- Email: [email protected]
Template-free synthesis of metallic WS2 hollow microspheres as an anode for the sodium-ion battery
Journal of Colloid and Interface Science 557 (2019) 722–728
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Template-free synthesis of metallic WS2 hollow microspheres as an anode for the sodium-ion battery Jianbiao Wang a,b, Ling Yu a,b, Ziwang Zhou a,b, Lingxing Zeng c, Mingdeng Wei a,b,d,⇑ a
Fujian Provincial Key Laboratory of Electrochemical Energy Storage Materials, Fuzhou University, Fuzhou, Fujian 350002, China State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China c College of Environment Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350007, China d Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou 213164, China b
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 7 August 2019 Revised 19 September 2019 Accepted 20 September 2019 Available online 21 September 2019 Keywords: WS2 Hollow microspheres Metallic feature Anode Sodium-ion battery Electrochemical properties
a b s t r a c t In this work, an electrode has been proposed and designed on its interior structure and controlled morphology, Specifically, metallic WS2 hollow microspheres have been synthesized via a one-step solvothermal process, in the absence of additives, consisting of porous nanosheets with a large surface area. The individual nanosheet features an expanded interlayer distance of 1.02 nm and metallic characteristics, which can provide faster transport of electrons and reduce the diffusion pathway for sodium ions. Furthermore, the effects of anions in the ether-based electrolyte on the cycling performance were also investigated and optimized. As a result, such an anode can demonstrate superior capacitive behavior with excellent rate performance (388.2 mA h g1 at 0.1 A g1, 307.7 mA h g1 at 1 A g1) and long-term cycling stability (285 mA h g1 at 2 A g1 after 2000 cycles). Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding author at: Fujian Provincial Key Laboratory of Electrochemical Energy Storage Materials, Fuzhou University, Fuzhou, Fujian 350002, China. E-mail address: [email protected] (M. Wei). https://doi.org/10.1016/j.jcis.2019.09.078 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
J. Wang et al. / Journal of Colloid and Interface Science 557 (2019) 722–728
1. Introduction Lithium ion batteries (LIBs) have been widely used in electronic devices due to their well-known advantages, but the further development of LIBs has been hampered by limited natural resources and high cost [1–3]. Sodium ion batteries (SIBs) have been intensively investigated and become a promising alternative to LIBs due to similar electrochemical kinetics performance, abundant natural resources, and low cost of sodium metal [4–9]. However, compared with LIBs, SIBs are often accompanied by sluggish electrochemical performance caused by the large radius of Na+. Therefore, to cope with the repeated insertion/extraction of Na+, exploring suitable anode materials with superior electrochemical performance has become imperative [10–17]. Among various anode materials, tungsten disulfide (WS2) has attracted much attention for SIBs, owing to the intrinsic characteristics of the large interlayer spacing (0.62 nm), high theoretical capacity, and weak van der Waals forces between interlayers [18,19]. It is known that WS2 shows some deficiencies, such as low electric conductivity, large volume variation, and aggregation in the charge/discharge process [20,21], which are needed to be mitigated. To generate an anode with enhanced electrochemical performance, substantial efforts have been made with a focus on the integration with carbonaceous materials (i.e. graphene, carbon nanofibers, and carbon nanotubes) [22]. A WS2/CNT-rGO aerogel was prepared by Yang et al. via two steps of freeze-drying and post annealing, exhibiting a capacity of 252.9 mA h g1 at 0.2 A g1 after 100 cycles [23]. Wang et al. reported free-standing WS2 nanosheets supported by 3D carbon foam, which demonstrated a capacity of 268.4 mA h g1 at 2 A g1 [24]. However, there are still challenges to be solved. Firstly, WS2 synthesisi procedure is complicated, in which an annealing process or high temperature is necessary (>200 °C). Secondly, the initial Coulombic efficiency for WS2based hybrid anodes is low (<70%) [25]. In addition, it is noted that the aforementioned strategies are all modulated by exterior factors. To alleviate the disadvantages of these strategies, fabricating the electrode from the interior structure is a well-studied option [26–28]. In fact, WS2 has two different phases, due to the distinct arrangement of S atoms, which are separated into the 2H and 1T phases [29,30]. Noteworthy, 2H phase of WS2 is a semiconductor with a large bandgap, and the counterpart 1T phase possesses metallic characteristics [31]. To the best of our knowledge, however, there are few reports dealing with applications of metallic 1T-WS2 with expanded interlayer spacing in SIBs. Herein, the 1T phase of metallic WS2 hollow microspheres composed of porous nanosheets with expanded interlayer space of 1.02 nm were rationally designed and prepared through a facile template-free solvothermal process in absence of additive. Used as an anode in SIBs, it could deliver a large capacity of 353.2 mA h g1 after 80 cycles at 0.2 A g1. In term of long cycling performance, a capacity of 285 mA h g1 could be maintained even after 2000 cycles at 2 A g1. It is believed that such a strategy provides design avenues for the fabrication of other electrode materials from the exterior factors.
2. Experimental section 2.1. Preparation of metallic WS2 hollow microspheres As for the synthesis of 1T WS2 hollow spheres, 0.2 g of WCl6 was firstly dispersed in a mixed solution (20 ml of ethanol and 10 ml of isopropanol), stirring for about 10 min. Subsequently, 0.38 g of thioacetamide was added and stirred for 30 min to obtain a homogeneous solution. Then, the above mixture was transferred into 50 ml stainless autoclave and the reaction was performed at
723
200 °C for 24 h. Finally, the obtained black product was collected by centrifugation and washed with ethanol and deionized water for several times and finally dried in vacuum oven for overnight. 2.2. Material characterization The crystal structure of 1T metallic WS2 hollow spheres was analyzed by Raman spectrum (Horiba) with an excitation wavelength of 532 nm and X-ray diffraction analysis (Rigaku Ultima IV) with Cu radiation of 1.5418 Å. The metallic feature of the sample was characterized by UV–vis (LAMBDA800 PE). The surface chemical bonds of the as prepared sample were collected by Xray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250). The surface area and pore size distribution were evaluated on a Micromeritics ASAP 2020 instrument. The morphology and the interior structure of the prepared sample were explored by scanning electron microscopy (SEM, Hitachi S4800 instrument) and transmission electron microscopy (TEM, FEI F20 STWIN). 2.3. Electrochemical measurements To explore the electrochemical performance of metallic WS2 hollow spheres, CV curves at various sweep rates were conducted and electrochemical impedance spectra were investigated in the range of 105–0.1 Hz on electrochemical working station (Shanghai, Chenhua). CR2025 coin cells were assembled in glove box. In detail, the working electrode was composed of a mixture of active material, carbon black and PVDF in a weight ratio of 7:2:1. Metal sodium and glass fiber (Whatman) were regarded as counter electrode and separator, respectively. 1 M NaSO3CF3 in diglyme makes up the electrolyte. For comparison, the electrolyte composed of 1 M NaClO4 in diglyme was also applied. The cycling performance at different current rates were conducted on battery testing system (Land CT 2001A, China). 3. Results and discussion Fig. 1 presents the SEM and TEM images of the prepared sample. From the large scale image in Fig. 1a, it can be observed that the sample has a microspherical morphology with a diameter of 600 nm. A magnified SEM image from the incomplete microsphere in Fig. 1b demonstrates the existence of a hollow structure composed of curved nanosheets. More observations of SEM images were employed to verify the hollow sphere morphology, as shown in Fig. S1a, b. The typical TEM image in Fig. 1c also reveals the presence of a hollow interior structure, and the inner diameter of these hollow spheres were measured to be appropriately 400 nm. the high resolution TEM image shown in Fig. 1d demonstrates that the hollow microspheres are composed of a few layers of nanosheets with an expanded interlayer spacing of 0.98 nm, corresponding to the (0 0 2) plane. The elemental mapping in Fig. 1e–g depict homogeneous distribution of W and S through the whole hollow microsphere. To investigate the formation mechanism of WS2 hollow spheres, a series of time-dependent experiences were conducted. As shown in Fig. S2, with the time going on, the nanosheets started to assemble and finally formed the WS2 hollow microspheres. A possible schematic growth process was proposed in Fig. S3. Furthermore, XRD patterns were also conducted to reveal WS2 phase upon the solvothermal time. As depicted in Fig. S4, impurities have been observed in the beginning. When the reaction time was increased to 18 h, a pure phase of WS2 hollow spheres has been formed. Raman spectra and XRD patterns are demonstrated in Fig. 2 confirming the crystal structure of the WS2 hollow microspheres. As well known, the characteristic bands of 2H WS2 are located
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Fig. 1. (a, b) SEM images, (c, d) TEM images and (e-g) elemental mapping of W and S elements of WS2 hollow spheres.
around 350 and 420 cm1, corresponding to the in-plane E2g and out-of-plane A1g modes, respectively. Fig. 2a reveals the peaks at 129, 178, and 254 nm, verifying the existence and high purity of metallic 1T WS2. The XRD pattern in Fig. 2b depicts a shifted peak at 2h = 8.4°, corresponding to an expanded interlayer distance of 1.02 nm, in comparison with the standard peak at 2h = 14.3°. Furthermore, the additional small peaks observed around 16.4° and 27.6° are assigned to the of the (0 0 4) and (0 0 6) planes, respectively, which are also the typical characteristic peaks of 1T WS2 [30]. Noteworthy, an anode with expanded interlayer spacing can provide more active sites and accommodate more sodium ions in the charge/discharge process, and the metallic feature may result in superior rate performance.
As revealed in Fig. 3a, UV diffuse reflectance spectroscopy was performed in the range of 400–2500 nm. It can be found that there is no obvious absorption band edge, which indicates that the wavelength for the absorption band is beyond 2500 nm. Therefore, it can be calculated that the bandgap for 1T WS2 hollow microspheres is less than 0.5 eV, confirming the metallic feature of 1T WS2 hollow spheres. XPS analysis was conducted in the range of 0–1200 eV to collect the surface chemical states of W and S elements in the obtained sample. The full survey spectrum shown in Fig. 3b indicates the co-existence of W and S. Fig. 3c demonstrates three prominent peaks at 32.2, 34.3, and 35.8 eV. Compared with 2H WS2 in previous reports, they exhibit lower binding energies, indicating the successful synthesis of 1T WS2 [28,29]. The binding energy of 38.1 eV was assigned to partial oxidation on the surface, which is common in the preparation of WS2 nanomaterials. The high resolution spectrum of S 2p at 162 and 163 eV indicate the existence of S2- in WS2 hollow microspheres (Fig. 3d). The N2 adsorption-desorption isotherm was carried out (Fig. S5) to provide insights into the porous features of 1T WS2 hollow microspheres. It could be estimated that WS2 hollow spheres have a surface area of 46 m2 g1. Fig. S5 reveals that the pore size of metallic WS2 hollow spheres centers around 2 nm. Then, it is known that the larger surface area can supply sufficient contact area between the electrode and electrolyte and facilitate the transport of sodium ions, which also contribute to the superior cycling performance. The electrochemical performance of metallic WS2 hollow microspheres were explored, as shown in Fig. 4. The rate performance of WS2 hollow microspheres were demonstrated in Fig. 4a at stepwise incremental current densities from 0.05 to 1 A g1. The electrode could deliver capacities of 414.0, 388.2, 344.5, 319.6, and 307.7 mA h g1 at the current densities of 0.05, 0.1, 0.2, 0.5, and 1 A g1, respectively. When the current density was returned to 0.1 A g1, a capacity of 442.2 mA h g1 could be maintained, which is higher than the initial capacity, and may attribute to the unique layered structure. The cycling performance of metallic WS2 hollow microspheres at 0.2 A g1 was also shown in Fig. 4b, delivering a capacity of 353.2 mA h g1 after 80 cycles with an initial Coulomobic efficiency of 96.9%. As revealed in Fig. 4c, to explore the electrode stability at a large current density of 2 A g1, a capacity of 285 mA h g1 could be maintained even after 2000 cycles, with a capacity retention above 100% compared with the second cycle. The increasing capacity in the cycles may be attributed to the active process and the defects generated upon cycling [32–34]. CV curves at a sweep rate of 0.2 mV s1 are presented in Fig. 5a in the voltage window of 0.25–3 V to further investigate the sodi-
Fig. 2. Raman spectrum and XRD pattern of metallic WS2 hollow spheres.
J. Wang et al. / Journal of Colloid and Interface Science 557 (2019) 722–728
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Fig. 3. (a) UV diffuse reflectance absorption spectrum, (b) XPS survey spectrum, (c) W 4f, (d) S 2p XPS spectra of metallic WS2 hollow spheres.
Fig. 4. Electrochemical performances of metallic WS2 hollow spheres: (a) Rate performance, (b) cycling performance at 0.2 A g1, (c) long cycling performance at 2 A g1.
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Fig. 5. (a) CV curves at a scanning rate of 0.2 m V s1, (b) charge-discharge profiles at various current densities, (c) CV curves at increasing sweep rates from 0.2 to 2 mV s1 and (d) the corresponded linear relationship between log (i) and log (m) obtained from CV curves.
ation/desodiation process. In the first cathodic process, the electrode exhibits two peaks at 0.69 and 0.47 V, which are separately assigned to the insertion of sodium ions, the formation of solid electrolyte film as well as the conversion reaction from NaxWS2 to metallic W and Na2S. The corresponding anodic peaks at 1.52 and 2.12 V are derived from the multistep oxidation of W to WS2. From the second cycle, the reduction peak at 1.5 and 0.75 V were separately ascribed to the formation of NaxWS2 and the reduction of NaxWS2 to W and Na2S. The corresponding oxidation peaks at 1.6, 1.75, and 2.5 V might be attributed to the formation of NaxWS2 and extraction of Na+. Furthermore, the following cycles are mostly overlapped, indicating electrochemical stability of the metallic WS2 hollow microspheres. Fig. 5b shows the charge-discharge profiles at stepwise incremental current densities demonstrating low polarization even at a large current density. The profiles exhibit distinct platforms during the charge-discharge process, which were consistent with the CV results. Furthermore, to estimate the capacity contribution from the capacitance, CV curves at increasing sweep rates were conducted in Fig. 5c. It was calculated based on the following Eq. (1):
i ¼ amb
ð1Þ
where i represents the current, m is the sweep rate, and a, and b are adjustable parameters. Specifically, b determines the capacity contribution from capacitance or the diffusion-controlled process. When the value of b is close to 0.5, it means the current is primarily determined by a diffusion-controlled process. Conversely, the capacitance controls the current when the value of b is close to 1. As demonstrated in Fig. 5d, the b values from the oxidation and reduction peaks are 0.90 and 0.93, respectively, which reveals the electrochemical process was controlled by the capacitive effect. To further account for the high-rate performance, the capacitive contribution was qualified by the following equation:
i ¼ k1 m þ k2 m1=2
ð1Þ
where k1m represents the capacitive contribution and k2m1/2 represents the diffusion controlled process. As shown in Fig. 6a, it can be observed that the capacitive ratio increases with increasing scan rates. In detail, 90.5% of the capacity is derived from the capacitive effect at a sweep rate of 2 mV s1 (Fig. 6b). In contrast, to investigate the effects of anions in the electrolyte, as shown in Fig. 6c-d, the cycling performances were conducted in the contrast electrolyte (NaClO4 in diglyme) at current densities of 0.5 and 1 A g1, respectively. Inferior cycling stability could be observed in the charge–discharge process, indicative of the SO3CF-3 favoring the Coulombic efficiency and cycling stability. The electrochemical performance of other WS2-based materials are listed in Table 1. It is noticed that metallic WS2 hollow spheres show superior electrochemical performance compared with these carbonaceous hybrids, demonstrating the rational design of metallic 1 T WS2 hollow spheres [21,23,24,35–37]. Electrochemical impedance spectra were further employed to highlight the sodium storage performance. As demonstrated in Fig. 7, the spectra after the 1st cycle and 20th cycles were collected in the frequency range of 105–0.1 Hz. Simplified Randles equivalent circuit modes are depicted in the inset to qualify the fitted results of the metallic WS2 hollow microspheres. From the fitted result in Table S1, it could be concluded that the charge-transfer resistance is 8 X less after 20 cycles than after the 1st cycle. Furthermore, it is noteworthy that the slope is steeper after 20 cycles than after the 1st cycle, which indicates faster ion transport. Additionally, the EIS result might also account for the increasing capacities in the initial cycles. To provide an evidence for the stability of the electrode, SEM images of the electrode after 200 cycles were conducted. As shown in Fig. S6, it can be observed that the electrode can still maintain the hollow structure.
J. Wang et al. / Journal of Colloid and Interface Science 557 (2019) 722–728
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Fig. 6. (a) Contribution ratio of the capacitive and diffusion-controlled at different corresponded scan rates, (b) CV curves at a scan rate of 2 mV s1, (c, d) cycling performances at 0.5 A g1 and 1 A g1, respectively, for WS2 hollow spheres in the contrast electrolyte.
Table 1 The electrochemical performance of WS2-based anode materials for SIBs. Materials
Current density (A g1)
Capacity (mA h g1)
References
Cubic-shaped WS2 nanopetals
0.2
320 after 100 cycles 78 after 500 cycles 318.5 after 400 cycles 226.5 after 800 cycles 360 after 100 cycles 200 after 100 cycles 333 after 70 cycles 230 (rate capacity) 252.9 after 100 cycles 392.1 after 1000 cycles 268.4 (rate capacity) 353.2 after 80 cycles 285 after 2000 cycles
35
5 WS2 nanofibers
0.2 1
N-doped carbon/WS2 nanosheets
0.1 1
WS2/CMK-3
0.1 2
Hierarchically WS2/CNT-rGO 3D carbon foam supported WS2
0.2 0.2 2
WS2 hollow spheres
0.2 2
21
36
37
Fig. 7. EIS spectra of WS2 hollow spheres after 1st and 20 cycle, respectively. 23 24
This work
4. Conclusions 1T metallic WS2 hollow microspheres composed of nanosheets were synthesized via a one-step solvothermal process in the absence of additives. The presence of nanosheets in the resulting
sample could effectively shorten the diffusion pathway of the sodium ions. On the other hand, the interior hollow structure also provides a large surface area for the volume expansion in the sodiation/desodiation process. In addition to these exterior morphological factors, the intrinsic structure properties of the metallic features and the expanded interlayer spacing for the obtained nanosheets also have a significant effect on the ultimate outstanding electrochemical performance, especially for the high rate performance. In detail, it demonstrates superior capacitive behavior, demonstrating a capacity of 353.2 mA h g1 after 80 cycles at 0.2 A g1, and a long-term cycling performance of 285 m A h g1 after 2000 cycles at 2 A g1. It is believed that the combining engineering designs of interior structure and morphology will inspire the construction of other transition metal materials.
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Acknowledgement This work was supported by the National Natural Science Foundation of China (NSFC U1505241, NSFC 51502036) and Natural Science Foundation of Distinguished Young Scholars for Fujian Province (2019J06015). Appendix A. Supplementary material
[15] [16] [17] [18] [19] [20] [21] [22]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.09.078.
[23] [24]
References [1] Z. Li, J.T. Zhang, Y. Lu, X.W. Lou, Sci. Adv. 4 (2018) 1687. [2] Y. Liu, A.A. Elzatahry, W. Luo, K. Lan, P.F. Zhang, J.W. Fan, Y. Wei, C. Wang, Y.H. Deng, G.F. Zheng, F. Zhang, Y. Tang, L.Q. Mai, D.Y. Zhao, Nano Energy 25 (2016) 80–90. [3] J. Wang, Z. Liu, W. Yang, L. Han, M. Wei, Chem. Commun. 54 (2018) 7346– 7349. [4] J.B. Wang, N.J. Luo, J.X. Wu, S.P. Huang, L. Yu, M.D. Wei, J. Mater. Chem. A 7 (2019) 3691–3696. [5] X. Wang, Y. Chen, Y.J. Fang, J.T. Zhang, S.Y. Gao, X.W. Lou, Angew. Chem. Int. Edit. 58 (2019) 2675–2679. [6] Y. Liu, Y.J. Fang, Z.W. Zhao, C.Z. Yuan, X.W. Lou, Adv. Energy Mater. 9 (2019) 1803052. [7] Z. Li, K. Jiang, F. Khan, A. Goswami, J. Liu, A. Passian, T. Thundat, Sci. Adv. 5 (2019) 2820. [8] D. Sun, D.L. Ye, P. Liu, Y.G. Tang, J. Guo, L.Z. Wang, H.Y. Wang, Adv. Energy Mater. 8 (2018) 1702383. [9] Y.X. Hu, B. Luo, D.L. Ye, X.B. Zhu, M.Q. Lyu, L.Z. Wang, Adv. Mater. 29 (2017) 1606132. [10] Z.Y. Zhang, S.L. Wu, J.Y. Cheng, W.J. Zhang, Energy Storage Mater. 15 (2018) 65–74. [11] J.B. Wang, L. Chen, L.X. Zeng, Q.H. Wei, M.D. Wei, A.C.S. Sustain, Chem. Eng. 6 (2018) 4688–4694. [12] J. Wang, L. Han, X. Li, L. Zeng, M. Wei, J. Colloid Interf. Sci. 548 (2019) 20–24. [13] L. Zeng, F. Luo, X. Xia, M.Q. Yang, L. Xu, J. Wang, X. Feng, Q. Qian, M. Wei, Q. Chen, Chem. Commun. 55 (2019) 3614–3617. [14] Z.W. Li, W.J. Feng, Y.Q. Lin, X. Liu, H.L. Fei, RSC Adv. 6 (2016) 70632–70637.
[25] [26] [27] [28] [29] [30]
[31] [32] [33] [34] [35] [36] [37]
H.L. Fei, Y.Q. Lin, J. Colloid Interf. Sci. 481 (2016) 256–262. H.L. Fei, W.J. Feng, T. Xu, J. Colloid Interf. Sci. 488 (2017) 277–281. H.L. Fei, Z.W. Feng, X. Liu, Ionics 21 (2015) 1967–1972. Y.C. Song, J.X. Liao, C. Chen, J. Yang, J.C. Chen, F. Gong, S.Z. Wang, Z.Q. Xu, M.Q. Wu, Carbon 142 (2019) 697–706. Y. Von Lim, Y. Wang, D.Z. Kong, L. Guo, J.I. Wong, L.K. Ang, H.Y. Yang, J. Mater. Chem. A 5 (2017) 10406–10415. C.B. Zhu, P. Kopold, W.H. Li, P.A. van Aken, J. Maier, Y. Yu, J. Mater. Chem. A 3 (2015) 20487–20493. X.D. Xu, X.J. Li, J.Q. Zhang, K. Qiao, D.Z. Han, S.T. Wei, W. Xing, Z.F. Yan, Electrochim. Acta 302 (2019) 259–269. J.X. Wu, Z.H. Lu, K.K. Li, J. Cui, S.S. Yao, M.I.U. Haq, B.H. Li, Q.H. Yang, F.Y. Kang, F. Ciucci, J.K. Kim, J. Mater. Chem. A 6 (2018) 5668–5677. Y. Wang, D.Z. Kong, W.H. Shi, B. Liu, G.J. Sim, Q. Ge, H.Y. Yang, Adv. Energy Mater. 6 (2016) 1601057. Y. Wang, D.Z. Kong, S.Z. Huang, Y.M. Shi, M. Ding, Y.V. Lim, T.T. Xu, F.M. Chen, X.J. Li, H.Y. Yang, J. Mater. Chem. A 6 (2018) 10813–10824. S.H. Choi, Y.C. Kang, Nanoscale 7 (2015) 3965–3970. W.J. Tang, X.L. Wang, D. Xie, X.H. Xia, C.D. Gu, J.P. Tu, J. Mater. Chem. A 6 (2018) 18318–18324. P. He, M. Yan, G. Zhang, R. Sun, L. Chen, Q. An, L. Mai, Adv. Energy Mater. 7 (2017) 1601920. Y. Liu, N. Zhang, H. Kang, M. Shang, L. Jiao, J. Chen, Chem-Eur. J. 21 (2015) (1884) 11878–11881. B. Mahler, V. Hoepfner, K. Liao, G.A. Ozin, J. Am. Chem. Soc. 136 (2014) 14121– 14127. Q. Liu, X. Li, Z. Xiao, Y. Zhou, H. Chen, A. Khalil, T. Xiang, J. Xu, W. Chu, X. Wu, J. Yang, C. Wang, Y. Xiong, C. Jin, P.M. Ajayan, L. Song, Adv. Mater. 27 (2015) 4837–4844. J. Yi, X. She, Y. Song, M. Mao, K. Xia, Y. Xu, Z. Mo, J. Wu, H. Xu, H. Li, Chem. Eng. J. 335 (2018) 282–289. W. Qin, T.Q. Chen, L.K. Pan, L.Y. Niu, B.W. Hu, D.S. Li, J.L. Li, Z. Sun, Electrochim. Acta 153 (2015) 55–61. J.L. Li, W. Qin, J.P. Xie, R. Lin, Z.L. Wang, L.K. Pan, W.J. Mai, Chem. Eng. J. 332 (2018) 260–266. J. Li, J. Li, Z. Ding, X. Zhang, Y. Li, T. Lu, Y. Yao, W. Mai, L. Pan, Chem. Eng. J. 378 (2019) 122108. Y.V. Lim, Y. Wang, D. Kong, L. Guo, J.I. Wong, L.K. Ang, H.Y. Yang, J. Mater. Chem. A 5 (2017) 10406–10415. X. Wang, J. Huang, J. Li, L. Cao, W. Hao, Z. Xu, A.C.S. Appl, Mater. Inter. 8 (2016) 23899–23908. Q. Pang, Y. Gao, Y. Zhao, Y. Ju, H. Qiu, Y. Wei, B. Liu, B. Zou, F. Du, G. Chen, Chem-Eur. J. 23 (2017) 7074–7080.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Template-free synthesis of metallic WS2 hollow microspheres as an anode for the sodium-ion battery Jianbiao Wang a,b, Ling Yu a,b, Ziwang Zhou a,b, Lingxing Zeng c, Mingdeng Wei a,b,d,⇑ a
Fujian Provincial Key Laboratory of Electrochemical Energy Storage Materials, Fuzhou University, Fuzhou, Fujian 350002, China State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China c College of Environment Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350007, China d Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou 213164, China b
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 7 August 2019 Revised 19 September 2019 Accepted 20 September 2019 Available online 21 September 2019 Keywords: WS2 Hollow microspheres Metallic feature Anode Sodium-ion battery Electrochemical properties
a b s t r a c t In this work, an electrode has been proposed and designed on its interior structure and controlled morphology, Specifically, metallic WS2 hollow microspheres have been synthesized via a one-step solvothermal process, in the absence of additives, consisting of porous nanosheets with a large surface area. The individual nanosheet features an expanded interlayer distance of 1.02 nm and metallic characteristics, which can provide faster transport of electrons and reduce the diffusion pathway for sodium ions. Furthermore, the effects of anions in the ether-based electrolyte on the cycling performance were also investigated and optimized. As a result, such an anode can demonstrate superior capacitive behavior with excellent rate performance (388.2 mA h g1 at 0.1 A g1, 307.7 mA h g1 at 1 A g1) and long-term cycling stability (285 mA h g1 at 2 A g1 after 2000 cycles). Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding author at: Fujian Provincial Key Laboratory of Electrochemical Energy Storage Materials, Fuzhou University, Fuzhou, Fujian 350002, China. E-mail address: [email protected] (M. Wei). https://doi.org/10.1016/j.jcis.2019.09.078 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
J. Wang et al. / Journal of Colloid and Interface Science 557 (2019) 722–728
1. Introduction Lithium ion batteries (LIBs) have been widely used in electronic devices due to their well-known advantages, but the further development of LIBs has been hampered by limited natural resources and high cost [1–3]. Sodium ion batteries (SIBs) have been intensively investigated and become a promising alternative to LIBs due to similar electrochemical kinetics performance, abundant natural resources, and low cost of sodium metal [4–9]. However, compared with LIBs, SIBs are often accompanied by sluggish electrochemical performance caused by the large radius of Na+. Therefore, to cope with the repeated insertion/extraction of Na+, exploring suitable anode materials with superior electrochemical performance has become imperative [10–17]. Among various anode materials, tungsten disulfide (WS2) has attracted much attention for SIBs, owing to the intrinsic characteristics of the large interlayer spacing (0.62 nm), high theoretical capacity, and weak van der Waals forces between interlayers [18,19]. It is known that WS2 shows some deficiencies, such as low electric conductivity, large volume variation, and aggregation in the charge/discharge process [20,21], which are needed to be mitigated. To generate an anode with enhanced electrochemical performance, substantial efforts have been made with a focus on the integration with carbonaceous materials (i.e. graphene, carbon nanofibers, and carbon nanotubes) [22]. A WS2/CNT-rGO aerogel was prepared by Yang et al. via two steps of freeze-drying and post annealing, exhibiting a capacity of 252.9 mA h g1 at 0.2 A g1 after 100 cycles [23]. Wang et al. reported free-standing WS2 nanosheets supported by 3D carbon foam, which demonstrated a capacity of 268.4 mA h g1 at 2 A g1 [24]. However, there are still challenges to be solved. Firstly, WS2 synthesisi procedure is complicated, in which an annealing process or high temperature is necessary (>200 °C). Secondly, the initial Coulombic efficiency for WS2based hybrid anodes is low (<70%) [25]. In addition, it is noted that the aforementioned strategies are all modulated by exterior factors. To alleviate the disadvantages of these strategies, fabricating the electrode from the interior structure is a well-studied option [26–28]. In fact, WS2 has two different phases, due to the distinct arrangement of S atoms, which are separated into the 2H and 1T phases [29,30]. Noteworthy, 2H phase of WS2 is a semiconductor with a large bandgap, and the counterpart 1T phase possesses metallic characteristics [31]. To the best of our knowledge, however, there are few reports dealing with applications of metallic 1T-WS2 with expanded interlayer spacing in SIBs. Herein, the 1T phase of metallic WS2 hollow microspheres composed of porous nanosheets with expanded interlayer space of 1.02 nm were rationally designed and prepared through a facile template-free solvothermal process in absence of additive. Used as an anode in SIBs, it could deliver a large capacity of 353.2 mA h g1 after 80 cycles at 0.2 A g1. In term of long cycling performance, a capacity of 285 mA h g1 could be maintained even after 2000 cycles at 2 A g1. It is believed that such a strategy provides design avenues for the fabrication of other electrode materials from the exterior factors.
2. Experimental section 2.1. Preparation of metallic WS2 hollow microspheres As for the synthesis of 1T WS2 hollow spheres, 0.2 g of WCl6 was firstly dispersed in a mixed solution (20 ml of ethanol and 10 ml of isopropanol), stirring for about 10 min. Subsequently, 0.38 g of thioacetamide was added and stirred for 30 min to obtain a homogeneous solution. Then, the above mixture was transferred into 50 ml stainless autoclave and the reaction was performed at
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200 °C for 24 h. Finally, the obtained black product was collected by centrifugation and washed with ethanol and deionized water for several times and finally dried in vacuum oven for overnight. 2.2. Material characterization The crystal structure of 1T metallic WS2 hollow spheres was analyzed by Raman spectrum (Horiba) with an excitation wavelength of 532 nm and X-ray diffraction analysis (Rigaku Ultima IV) with Cu radiation of 1.5418 Å. The metallic feature of the sample was characterized by UV–vis (LAMBDA800 PE). The surface chemical bonds of the as prepared sample were collected by Xray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250). The surface area and pore size distribution were evaluated on a Micromeritics ASAP 2020 instrument. The morphology and the interior structure of the prepared sample were explored by scanning electron microscopy (SEM, Hitachi S4800 instrument) and transmission electron microscopy (TEM, FEI F20 STWIN). 2.3. Electrochemical measurements To explore the electrochemical performance of metallic WS2 hollow spheres, CV curves at various sweep rates were conducted and electrochemical impedance spectra were investigated in the range of 105–0.1 Hz on electrochemical working station (Shanghai, Chenhua). CR2025 coin cells were assembled in glove box. In detail, the working electrode was composed of a mixture of active material, carbon black and PVDF in a weight ratio of 7:2:1. Metal sodium and glass fiber (Whatman) were regarded as counter electrode and separator, respectively. 1 M NaSO3CF3 in diglyme makes up the electrolyte. For comparison, the electrolyte composed of 1 M NaClO4 in diglyme was also applied. The cycling performance at different current rates were conducted on battery testing system (Land CT 2001A, China). 3. Results and discussion Fig. 1 presents the SEM and TEM images of the prepared sample. From the large scale image in Fig. 1a, it can be observed that the sample has a microspherical morphology with a diameter of 600 nm. A magnified SEM image from the incomplete microsphere in Fig. 1b demonstrates the existence of a hollow structure composed of curved nanosheets. More observations of SEM images were employed to verify the hollow sphere morphology, as shown in Fig. S1a, b. The typical TEM image in Fig. 1c also reveals the presence of a hollow interior structure, and the inner diameter of these hollow spheres were measured to be appropriately 400 nm. the high resolution TEM image shown in Fig. 1d demonstrates that the hollow microspheres are composed of a few layers of nanosheets with an expanded interlayer spacing of 0.98 nm, corresponding to the (0 0 2) plane. The elemental mapping in Fig. 1e–g depict homogeneous distribution of W and S through the whole hollow microsphere. To investigate the formation mechanism of WS2 hollow spheres, a series of time-dependent experiences were conducted. As shown in Fig. S2, with the time going on, the nanosheets started to assemble and finally formed the WS2 hollow microspheres. A possible schematic growth process was proposed in Fig. S3. Furthermore, XRD patterns were also conducted to reveal WS2 phase upon the solvothermal time. As depicted in Fig. S4, impurities have been observed in the beginning. When the reaction time was increased to 18 h, a pure phase of WS2 hollow spheres has been formed. Raman spectra and XRD patterns are demonstrated in Fig. 2 confirming the crystal structure of the WS2 hollow microspheres. As well known, the characteristic bands of 2H WS2 are located
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Fig. 1. (a, b) SEM images, (c, d) TEM images and (e-g) elemental mapping of W and S elements of WS2 hollow spheres.
around 350 and 420 cm1, corresponding to the in-plane E2g and out-of-plane A1g modes, respectively. Fig. 2a reveals the peaks at 129, 178, and 254 nm, verifying the existence and high purity of metallic 1T WS2. The XRD pattern in Fig. 2b depicts a shifted peak at 2h = 8.4°, corresponding to an expanded interlayer distance of 1.02 nm, in comparison with the standard peak at 2h = 14.3°. Furthermore, the additional small peaks observed around 16.4° and 27.6° are assigned to the of the (0 0 4) and (0 0 6) planes, respectively, which are also the typical characteristic peaks of 1T WS2 [30]. Noteworthy, an anode with expanded interlayer spacing can provide more active sites and accommodate more sodium ions in the charge/discharge process, and the metallic feature may result in superior rate performance.
As revealed in Fig. 3a, UV diffuse reflectance spectroscopy was performed in the range of 400–2500 nm. It can be found that there is no obvious absorption band edge, which indicates that the wavelength for the absorption band is beyond 2500 nm. Therefore, it can be calculated that the bandgap for 1T WS2 hollow microspheres is less than 0.5 eV, confirming the metallic feature of 1T WS2 hollow spheres. XPS analysis was conducted in the range of 0–1200 eV to collect the surface chemical states of W and S elements in the obtained sample. The full survey spectrum shown in Fig. 3b indicates the co-existence of W and S. Fig. 3c demonstrates three prominent peaks at 32.2, 34.3, and 35.8 eV. Compared with 2H WS2 in previous reports, they exhibit lower binding energies, indicating the successful synthesis of 1T WS2 [28,29]. The binding energy of 38.1 eV was assigned to partial oxidation on the surface, which is common in the preparation of WS2 nanomaterials. The high resolution spectrum of S 2p at 162 and 163 eV indicate the existence of S2- in WS2 hollow microspheres (Fig. 3d). The N2 adsorption-desorption isotherm was carried out (Fig. S5) to provide insights into the porous features of 1T WS2 hollow microspheres. It could be estimated that WS2 hollow spheres have a surface area of 46 m2 g1. Fig. S5 reveals that the pore size of metallic WS2 hollow spheres centers around 2 nm. Then, it is known that the larger surface area can supply sufficient contact area between the electrode and electrolyte and facilitate the transport of sodium ions, which also contribute to the superior cycling performance. The electrochemical performance of metallic WS2 hollow microspheres were explored, as shown in Fig. 4. The rate performance of WS2 hollow microspheres were demonstrated in Fig. 4a at stepwise incremental current densities from 0.05 to 1 A g1. The electrode could deliver capacities of 414.0, 388.2, 344.5, 319.6, and 307.7 mA h g1 at the current densities of 0.05, 0.1, 0.2, 0.5, and 1 A g1, respectively. When the current density was returned to 0.1 A g1, a capacity of 442.2 mA h g1 could be maintained, which is higher than the initial capacity, and may attribute to the unique layered structure. The cycling performance of metallic WS2 hollow microspheres at 0.2 A g1 was also shown in Fig. 4b, delivering a capacity of 353.2 mA h g1 after 80 cycles with an initial Coulomobic efficiency of 96.9%. As revealed in Fig. 4c, to explore the electrode stability at a large current density of 2 A g1, a capacity of 285 mA h g1 could be maintained even after 2000 cycles, with a capacity retention above 100% compared with the second cycle. The increasing capacity in the cycles may be attributed to the active process and the defects generated upon cycling [32–34]. CV curves at a sweep rate of 0.2 mV s1 are presented in Fig. 5a in the voltage window of 0.25–3 V to further investigate the sodi-
Fig. 2. Raman spectrum and XRD pattern of metallic WS2 hollow spheres.
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Fig. 3. (a) UV diffuse reflectance absorption spectrum, (b) XPS survey spectrum, (c) W 4f, (d) S 2p XPS spectra of metallic WS2 hollow spheres.
Fig. 4. Electrochemical performances of metallic WS2 hollow spheres: (a) Rate performance, (b) cycling performance at 0.2 A g1, (c) long cycling performance at 2 A g1.
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Fig. 5. (a) CV curves at a scanning rate of 0.2 m V s1, (b) charge-discharge profiles at various current densities, (c) CV curves at increasing sweep rates from 0.2 to 2 mV s1 and (d) the corresponded linear relationship between log (i) and log (m) obtained from CV curves.
ation/desodiation process. In the first cathodic process, the electrode exhibits two peaks at 0.69 and 0.47 V, which are separately assigned to the insertion of sodium ions, the formation of solid electrolyte film as well as the conversion reaction from NaxWS2 to metallic W and Na2S. The corresponding anodic peaks at 1.52 and 2.12 V are derived from the multistep oxidation of W to WS2. From the second cycle, the reduction peak at 1.5 and 0.75 V were separately ascribed to the formation of NaxWS2 and the reduction of NaxWS2 to W and Na2S. The corresponding oxidation peaks at 1.6, 1.75, and 2.5 V might be attributed to the formation of NaxWS2 and extraction of Na+. Furthermore, the following cycles are mostly overlapped, indicating electrochemical stability of the metallic WS2 hollow microspheres. Fig. 5b shows the charge-discharge profiles at stepwise incremental current densities demonstrating low polarization even at a large current density. The profiles exhibit distinct platforms during the charge-discharge process, which were consistent with the CV results. Furthermore, to estimate the capacity contribution from the capacitance, CV curves at increasing sweep rates were conducted in Fig. 5c. It was calculated based on the following Eq. (1):
i ¼ amb
ð1Þ
where i represents the current, m is the sweep rate, and a, and b are adjustable parameters. Specifically, b determines the capacity contribution from capacitance or the diffusion-controlled process. When the value of b is close to 0.5, it means the current is primarily determined by a diffusion-controlled process. Conversely, the capacitance controls the current when the value of b is close to 1. As demonstrated in Fig. 5d, the b values from the oxidation and reduction peaks are 0.90 and 0.93, respectively, which reveals the electrochemical process was controlled by the capacitive effect. To further account for the high-rate performance, the capacitive contribution was qualified by the following equation:
i ¼ k1 m þ k2 m1=2
ð1Þ
where k1m represents the capacitive contribution and k2m1/2 represents the diffusion controlled process. As shown in Fig. 6a, it can be observed that the capacitive ratio increases with increasing scan rates. In detail, 90.5% of the capacity is derived from the capacitive effect at a sweep rate of 2 mV s1 (Fig. 6b). In contrast, to investigate the effects of anions in the electrolyte, as shown in Fig. 6c-d, the cycling performances were conducted in the contrast electrolyte (NaClO4 in diglyme) at current densities of 0.5 and 1 A g1, respectively. Inferior cycling stability could be observed in the charge–discharge process, indicative of the SO3CF-3 favoring the Coulombic efficiency and cycling stability. The electrochemical performance of other WS2-based materials are listed in Table 1. It is noticed that metallic WS2 hollow spheres show superior electrochemical performance compared with these carbonaceous hybrids, demonstrating the rational design of metallic 1 T WS2 hollow spheres [21,23,24,35–37]. Electrochemical impedance spectra were further employed to highlight the sodium storage performance. As demonstrated in Fig. 7, the spectra after the 1st cycle and 20th cycles were collected in the frequency range of 105–0.1 Hz. Simplified Randles equivalent circuit modes are depicted in the inset to qualify the fitted results of the metallic WS2 hollow microspheres. From the fitted result in Table S1, it could be concluded that the charge-transfer resistance is 8 X less after 20 cycles than after the 1st cycle. Furthermore, it is noteworthy that the slope is steeper after 20 cycles than after the 1st cycle, which indicates faster ion transport. Additionally, the EIS result might also account for the increasing capacities in the initial cycles. To provide an evidence for the stability of the electrode, SEM images of the electrode after 200 cycles were conducted. As shown in Fig. S6, it can be observed that the electrode can still maintain the hollow structure.
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Fig. 6. (a) Contribution ratio of the capacitive and diffusion-controlled at different corresponded scan rates, (b) CV curves at a scan rate of 2 mV s1, (c, d) cycling performances at 0.5 A g1 and 1 A g1, respectively, for WS2 hollow spheres in the contrast electrolyte.
Table 1 The electrochemical performance of WS2-based anode materials for SIBs. Materials
Current density (A g1)
Capacity (mA h g1)
References
Cubic-shaped WS2 nanopetals
0.2
320 after 100 cycles 78 after 500 cycles 318.5 after 400 cycles 226.5 after 800 cycles 360 after 100 cycles 200 after 100 cycles 333 after 70 cycles 230 (rate capacity) 252.9 after 100 cycles 392.1 after 1000 cycles 268.4 (rate capacity) 353.2 after 80 cycles 285 after 2000 cycles
35
5 WS2 nanofibers
0.2 1
N-doped carbon/WS2 nanosheets
0.1 1
WS2/CMK-3
0.1 2
Hierarchically WS2/CNT-rGO 3D carbon foam supported WS2
0.2 0.2 2
WS2 hollow spheres
0.2 2
21
36
37
Fig. 7. EIS spectra of WS2 hollow spheres after 1st and 20 cycle, respectively. 23 24
This work
4. Conclusions 1T metallic WS2 hollow microspheres composed of nanosheets were synthesized via a one-step solvothermal process in the absence of additives. The presence of nanosheets in the resulting
sample could effectively shorten the diffusion pathway of the sodium ions. On the other hand, the interior hollow structure also provides a large surface area for the volume expansion in the sodiation/desodiation process. In addition to these exterior morphological factors, the intrinsic structure properties of the metallic features and the expanded interlayer spacing for the obtained nanosheets also have a significant effect on the ultimate outstanding electrochemical performance, especially for the high rate performance. In detail, it demonstrates superior capacitive behavior, demonstrating a capacity of 353.2 mA h g1 after 80 cycles at 0.2 A g1, and a long-term cycling performance of 285 m A h g1 after 2000 cycles at 2 A g1. It is believed that the combining engineering designs of interior structure and morphology will inspire the construction of other transition metal materials.
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Acknowledgement This work was supported by the National Natural Science Foundation of China (NSFC U1505241, NSFC 51502036) and Natural Science Foundation of Distinguished Young Scholars for Fujian Province (2019J06015). Appendix A. Supplementary material
[15] [16] [17] [18] [19] [20] [21] [22]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.09.078.
[23] [24]
References [1] Z. Li, J.T. Zhang, Y. Lu, X.W. Lou, Sci. Adv. 4 (2018) 1687. [2] Y. Liu, A.A. Elzatahry, W. Luo, K. Lan, P.F. Zhang, J.W. Fan, Y. Wei, C. Wang, Y.H. Deng, G.F. Zheng, F. Zhang, Y. Tang, L.Q. Mai, D.Y. Zhao, Nano Energy 25 (2016) 80–90. [3] J. Wang, Z. Liu, W. Yang, L. Han, M. Wei, Chem. Commun. 54 (2018) 7346– 7349. [4] J.B. Wang, N.J. Luo, J.X. Wu, S.P. Huang, L. Yu, M.D. Wei, J. Mater. Chem. A 7 (2019) 3691–3696. [5] X. Wang, Y. Chen, Y.J. Fang, J.T. Zhang, S.Y. Gao, X.W. Lou, Angew. Chem. Int. Edit. 58 (2019) 2675–2679. [6] Y. Liu, Y.J. Fang, Z.W. Zhao, C.Z. Yuan, X.W. Lou, Adv. Energy Mater. 9 (2019) 1803052. [7] Z. Li, K. Jiang, F. Khan, A. Goswami, J. Liu, A. Passian, T. Thundat, Sci. Adv. 5 (2019) 2820. [8] D. Sun, D.L. Ye, P. Liu, Y.G. Tang, J. Guo, L.Z. Wang, H.Y. Wang, Adv. Energy Mater. 8 (2018) 1702383. [9] Y.X. Hu, B. Luo, D.L. Ye, X.B. Zhu, M.Q. Lyu, L.Z. Wang, Adv. Mater. 29 (2017) 1606132. [10] Z.Y. Zhang, S.L. Wu, J.Y. Cheng, W.J. Zhang, Energy Storage Mater. 15 (2018) 65–74. [11] J.B. Wang, L. Chen, L.X. Zeng, Q.H. Wei, M.D. Wei, A.C.S. Sustain, Chem. Eng. 6 (2018) 4688–4694. [12] J. Wang, L. Han, X. Li, L. Zeng, M. Wei, J. Colloid Interf. Sci. 548 (2019) 20–24. [13] L. Zeng, F. Luo, X. Xia, M.Q. Yang, L. Xu, J. Wang, X. Feng, Q. Qian, M. Wei, Q. Chen, Chem. Commun. 55 (2019) 3614–3617. [14] Z.W. Li, W.J. Feng, Y.Q. Lin, X. Liu, H.L. Fei, RSC Adv. 6 (2016) 70632–70637.
[25] [26] [27] [28] [29] [30]
[31] [32] [33] [34] [35] [36] [37]
H.L. Fei, Y.Q. Lin, J. Colloid Interf. Sci. 481 (2016) 256–262. H.L. Fei, W.J. Feng, T. Xu, J. Colloid Interf. Sci. 488 (2017) 277–281. H.L. Fei, Z.W. Feng, X. Liu, Ionics 21 (2015) 1967–1972. Y.C. Song, J.X. Liao, C. Chen, J. Yang, J.C. Chen, F. Gong, S.Z. Wang, Z.Q. Xu, M.Q. Wu, Carbon 142 (2019) 697–706. Y. Von Lim, Y. Wang, D.Z. Kong, L. Guo, J.I. Wong, L.K. Ang, H.Y. Yang, J. Mater. Chem. A 5 (2017) 10406–10415. C.B. Zhu, P. Kopold, W.H. Li, P.A. van Aken, J. Maier, Y. Yu, J. Mater. Chem. A 3 (2015) 20487–20493. X.D. Xu, X.J. Li, J.Q. Zhang, K. Qiao, D.Z. Han, S.T. Wei, W. Xing, Z.F. Yan, Electrochim. Acta 302 (2019) 259–269. J.X. Wu, Z.H. Lu, K.K. Li, J. Cui, S.S. Yao, M.I.U. Haq, B.H. Li, Q.H. Yang, F.Y. Kang, F. Ciucci, J.K. Kim, J. Mater. Chem. A 6 (2018) 5668–5677. Y. Wang, D.Z. Kong, W.H. Shi, B. Liu, G.J. Sim, Q. Ge, H.Y. Yang, Adv. Energy Mater. 6 (2016) 1601057. Y. Wang, D.Z. Kong, S.Z. Huang, Y.M. Shi, M. Ding, Y.V. Lim, T.T. Xu, F.M. Chen, X.J. Li, H.Y. Yang, J. Mater. Chem. A 6 (2018) 10813–10824. S.H. Choi, Y.C. Kang, Nanoscale 7 (2015) 3965–3970. W.J. Tang, X.L. Wang, D. Xie, X.H. Xia, C.D. Gu, J.P. Tu, J. Mater. Chem. A 6 (2018) 18318–18324. P. He, M. Yan, G. Zhang, R. Sun, L. Chen, Q. An, L. Mai, Adv. Energy Mater. 7 (2017) 1601920. Y. Liu, N. Zhang, H. Kang, M. Shang, L. Jiao, J. Chen, Chem-Eur. J. 21 (2015) (1884) 11878–11881. B. Mahler, V. Hoepfner, K. Liao, G.A. Ozin, J. Am. Chem. Soc. 136 (2014) 14121– 14127. Q. Liu, X. Li, Z. Xiao, Y. Zhou, H. Chen, A. Khalil, T. Xiang, J. Xu, W. Chu, X. Wu, J. Yang, C. Wang, Y. Xiong, C. Jin, P.M. Ajayan, L. Song, Adv. Mater. 27 (2015) 4837–4844. J. Yi, X. She, Y. Song, M. Mao, K. Xia, Y. Xu, Z. Mo, J. Wu, H. Xu, H. Li, Chem. Eng. J. 335 (2018) 282–289. W. Qin, T.Q. Chen, L.K. Pan, L.Y. Niu, B.W. Hu, D.S. Li, J.L. Li, Z. Sun, Electrochim. Acta 153 (2015) 55–61. J.L. Li, W. Qin, J.P. Xie, R. Lin, Z.L. Wang, L.K. Pan, W.J. Mai, Chem. Eng. J. 332 (2018) 260–266. J. Li, J. Li, Z. Ding, X. Zhang, Y. Li, T. Lu, Y. Yao, W. Mai, L. Pan, Chem. Eng. J. 378 (2019) 122108. Y.V. Lim, Y. Wang, D. Kong, L. Guo, J.I. Wong, L.K. Ang, H.Y. Yang, J. Mater. Chem. A 5 (2017) 10406–10415. X. Wang, J. Huang, J. Li, L. Cao, W. Hao, Z. Xu, A.C.S. Appl, Mater. Inter. 8 (2016) 23899–23908. Q. Pang, Y. Gao, Y. Zhao, Y. Ju, H. Qiu, Y. Wei, B. Liu, B. Zou, F. Du, G. Chen, Chem-Eur. J. 23 (2017) 7074–7080.